Shaped thin-walled trays and containers are in wide use, for example in the storage and service of food. They are ordinarily used once or only a few times before being discarded and thus must be inexpensive and easily manufactured. Three major types of such articles are in general use, i.e. those composed of relatively low melting thermoplastic materials (such as polyethylene, polypropylene, polystyrene, and polyvinyl chloride), of aluminum sheeting, and of thermoset resins (e.g., the melamines). Although all are widely used, each suffers from certain important deficiencies. Thus, the thermoplastic articles can be used over only limited temperature ranges (usually not higher than 100.degree. C), the aluminum articles tend to have sharp edges and to dent easily, and cannot be used in microwave apparatus. The thermoset resins, especially the widely used melamines, have relatively poor impact strength, are brittle, tend to give off odors when heated and are rather expensive.
Except for the aluminum articles, they are commonly manufactured by injection molding processes which require relatively high capital outlays for the injection machinery and precision dies. Recently, a more economical process has been devised for the preparation of shaped thin-walled polymeric articles. It is an electrostatic powder spraying process (see, for example, U.S. Pat. Nos. 3,278,656, 3,369,274 and 3,482,013) which involves releasing electrostatically charged powdered polymeric material in the region of a heated shaped mold adapted to attract the powder. The area at the edge of the mold surface is of an insulating material which rejects the charged particles. Upon contacting the heated surface of the mold, the powder melts, flows into a continuous phase and solidifies when the mold cavity is subsequently cooled (all without the application of pressure). This process has proved to be quite successful in producing relatively satisfactory articles at very low costs.
The electrostatic spray process has, however, been used successfully only with polymeric materials which soften at relatively low temperatures such as polyethylene and polyvinyl chloride. Polymeric materials which are stable at higher temperature, such as polyethylene terephthalate, have not been used successfully in the electrostatic spray process. In fact it has not, insofar as is known, been possible heretofore to prepare ovenable articles utilizing this process.